1. Field of the Invention
The present invention relates generally to a passenger restraint device such as an air-bag, and more specifically to a control circuit therefore which features a highly reliable control circuit configuration.
2. Description of the Prior Art
FIG. 1 shows a prior art air bag control circuit arrangement which is comprised of a DC source 51, a malfunction inhibitor sensor 52 detonators 53 and 54, and an impact sensor (acceleration switch) arrangement which includes two switches 55 and 56 connected in parallel.
The malfunction inhibitor switch can take the form of a mercury switch or the like which is responsive to vehicle motion and indicates if the vehicle is at standstill or not.
With this arrangement, in the event of a collision, if the either of the impact sensor switches 55 and 56 are closed (ON) at the same time as the malfunction inhibitor sensor 52 is ON, direct current is supplied from the DC source 51 to the detonators 53 and 54 with the result that a restraining device such as an air bag, is rapidly deployed/activated.
However, this arrangement suffers from the drawbacks that as the impact sensor switches 55 and 56 are of the mechanical type, they must precisely made (which increases the cost) and even when precisely manufactured tend not to provide the required level of reliability.
To overcome these problems the arrangement shown in FIG. 2 has been proposed. This arrangement is such that the mechanical switch arrangement is replaced with a semi-conductor type acceleration sensor 67, an impact discrimination circuit 68 and a switching arrangement generally denoted by the numeral 613. In this instance the switching arrangement 613 includes two switching transistors 69 and 610 and two fixed current sources 611 and 612. The switching transistors and the current sources are paired and arranged in series with the detonators 63 and 64 in the illustrated manner.
The impact discrimination circuit 68 is arranged to determine, based on the analog signal output by the acceleration sensor 67, if a collision which is apt to induce physical harm or death has occurred or not. In the event of an affirmative decision, the circuit 68 applies a voltage to the gates of the switching transistors in a manner which render the same conductive (viz., ON). The fixed current sources 611, 612 respond by causing currents to pass through the detonators 63 and 64 and thus induces the deployment of the air-bag or activation of the like type of restraining device.
However, this arrangement suffers from the drawback that, should the impact be of such a nature as to cause the section of wiring indicated by A, to be severed or the insulative wire coating removed in a manner which permits grounding to take place, even though the switching transistors 69 and 610 may be rendered conductive, the connection between the DC source 61 and the detonators 63 and 64 has been cut or deteriorated to the point that neither can be ignited. This, of course, renders the air-bag or like type of restraint device inoperative.
On the other hand, in the event that the sections of wiring B and C between the detonators 63, 64 and the switching transistor 69 and 610 should become grounded by the removal of the insulating covering or the like, as soon as the malfunction inhibitor switch 62 assumes a closed (ON) condition (such as can be caused by vibration induced chatter), it becomes possible for direct current to flow through one or both of the detonators 3 and 4. This of course results a highly unexpected and totally erroneous deployment of the air bag.
FIG. 3 shows another example of air bag control circuitry. This arrangement is disclosed in jP-A-49-55031. In this arrangement AB denotes an air-bag which is operatively connected with a impact detection type accelerometer or G sensor 71 via a timing circuit arrangement. In this instance the timing circuit arrangement includes an amplifier 72; a first comparator 74 which compares the output of the amplifier with a first predetermined slice level S1 and acts as a switch; an integrator 76; a second slice level comparator type switch circuit 78, and a pulse generator 710 which is operatively connected with an igniter or squib 712. The latter mentioned element of course being used to detonate a charge which induces the required rapid gas generation.
With this arrangement, the output of the G sensor 71 is amplified, and produces a signal which contains a DC component. When this DC component containing signal exceeds the first slice level S1 in comparator 74, the device switches and the output is supplied to the integrator 76 which integrates the DC component and supplies the result to the second comparator 78. When the integrated value exceeds a second slice level SK, the second comparator 78 switches and produces an output which is supplied to the pulse generator 710, which in turn induces the ignition of the air bag inflation charge.
FIG. 4 shows a second arrangement which is disclosed in the above mentioned document. In this arrangement an impact sensing G sensor 814 is operatively connected with an amplifier 816. The output of the amplifier 816 is connected to a slice level switch type circuit arrangement 818 which is arranged to output a signal in the event that the input exceeds a first slice level S1. A first integrator circuit 820 is operatively connected with the output terminals of both the amplifier 816 and the first slice level switch 818. A second integrator 822 is operatively connected with the output terminal of the first integrator 820 and the output terminal of a second slice level switch type circuit 824 which is arranged, as shown, to receive the output of the first integrator 820.
A third slice level switch circuit arrangement 826 is connected to the output of the second integrator 822 and arranged to compare the output thereof with a predetermined slice level VK. Upon the slice level being reached, the third slice level switch circuit 826 outputs a signal to a pulse generator 828 which responds by inducing the ignition of an air bag inflation charge.
The arrangements shown in FIGS. 3 and 4 suffer from the drawbacks that they are unable to adequately distinguish between accidents wherein the initial amount of damage is large and that wherein the initial damage is relatively small for a given period and then rapidly increases. Viz., in the case a vehicle collides directly against as solid wall and the deceleration to which the occupant is subjected increases rapidly, it is very easy to determine that deployment of an air bag is desirable.
However, in the case wherein the vehicle collides with a pole (e.g. a steel light pole 30-40 cm in diameter) it is highly likely that the pole will "cut" through the paneling and less resilient components of the vehicle and for a given short time causing localized deformation of the bumper, front panels, radiator etc., before coming into contact with the engine or the like rigid structure which will produce very rapid deceleration. In this type of accident it is therefore highly likely that vehicular deceleration is apt to remain at relatively low levels while the pole "cuts" through the front of the vehicle and then suddenly increase to a magnitude sufficient to endanger the life of the driver and/or other occupants. This renders it very difficult to determine just when to deploy an air bag. If the bag inflated too early, it will tend to be deflating when the passenger comes into contact with the same and thus not be able to provide the required cushioning and passenger movement attenuation. On the other hand, if the inflation of the bag is delayed, it will not be fully inflated at the time the maximum cushioning effect is required.
Therefore, there has hitherto been a demand for a highly reliable circuit which does not exhibit the tendency to produce erroneously timed activation trigger signals, which does not malfunction in response to wiring disconnections and the like, and which can be produced at a reasonable cost.